Duplication master suitable for the manufacture of electroplated copies

ABSTRACT

A master suitable for the manufacture of electroplated negative metal copies. The master comprises a preform which on at least one side has a layer of electroplated aluminum. The surface has a precisely machined profile and is coated with a titanium nitride layer.

BACKGROUND OF THE INVENTION

The invention relates to a master suitable for the manufacture ofelectroplated negative copies of metal.

The negative metal copies are hereinafter referred to as fathermatrices. The father matrices are usually manufactured from nickel orcopper and in turn are used for making further copies. For example, thenegative father matrices are used to manufacture positive metal copiesknown as mother matrices. Negative son matrices are manufactured fromthe positive mother matrices and are used to manufacture copies of themaster which are wholly synthetic resin or which have a synthetic resincoating. Such copies are, for example, information carriers such asphonograph records, or in particular optical components such as mirrors,lenses, prisms and the like.

The optical components preferably comprise a supporting member of, forexample, glass or quartz. A coating layer of, for example, a light-curedsynthetic resin is provided on one side of the supporting member. Thesurface of the coating layer is the negative of that of the son matrix.The negative father matrix may also be used for making positive copiesof a synthetic resin.

According to a current process of manufacturing a master for an opticalcomponent, the surface of an aluminum alloy disc is machined to thedesired shape, for example an aspherical shape. It is not possible,however, to make the surface of an aluminum alloy super smooth byprecision machining. The surface has irregularities in the form ofmicropits. The surface is not of optical quality.

Another serious disadvantage is that only one father matrix can bemanufactured from the master. For this purpose a metal peel, for examplea Ni peel or a Cu-peel, is grown on the treated surface of the master byelectroplating. The aluminum alloy is then dissolved, and hence themaster is lost.

A master for phonograph records is manufactured according to a knownmethod by providing a lacquer layer on one side of a supporting plate,for example a metal or glass plate. An information track is cut in thelacquer layer with a chisel.

The master is provided, on the side of the lacquer layer, with anelectrolessly deposited metal layer, for example a Ag layer. A Ag layercan be provided by a chemical metallization process (plating process) bytreating the surface, simultaneously or successively, with an aqueoussilver salt solution and an aqueous solution of a reducing agent.

A thicker metal layer, for example a Ni layer or a Cu layer, is providedby electroplating on the electrolessly deposited metal layer. With themaster plate is finally removed, the lacquer layer is destroyed.Therefore, in this method the master can be used only once.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a reusable master having anoptical quality surface having a surface roughness R_(T) of at most 0.02μm.

It is another object of the invention that the copies derived from themaster, for example the father matrix, the mother matrix, the sonmatrix, and the synthetic resin copies, have the same or substantiallythe same surface quality as the master.

These objects are achieved by a master comprising a preform which on atleast one side has a layer of electroplated aluminum. The aluminumsurface is precisely machined to a desired profile and is then coatedwith a layer of titanium nitride.

The preform is preferably metal. Examples of suitable metals are copperor an aluminum alloy. The preform may alternatively be made from adifferent material, for example glass, quartz or a synthetic resin.These latter materials do not conduct electricity. When nonconductivematerials are used, the surface of the preform on which the layer ofelectroplated aluminum is deposited must first be made electricallyconductive by providing thereon an intermediate metal layer byelectroless deposition. A suitable intermediate metal layer is a silverlayer provided by an electroless chemical metallization process or avapor-deposited or sputtered layer of, for example, aluminum, copper,silver, gold or alloys thereof.

The layer of electroplated aluminum is provided according to knownmethods. Such methods are described in, for example, Netherlands PatentApplication No. 7812062 (corresponding to U.S. Pat. No. 4,257,854), andthe article by van de Berg et al entitled "The electrodeposition ofaluminium" (Philips Technical Review, Vol. 39, No. 3/4, pages 87-91,1980). The thickness of the electroplated aluminum layer is notrestricted to narrow limits. It may be for example, between 10 and 300μm, in particular between 20 and 100 μm.

The surface of the electroplated aluminum layer has a profile which isidentical to the desired profile of the synthetic resin products whichare manufactured by the father matrix derived from the master. Forexample, the surface of a master for photograph records comprises aspiral-like information track provided by a cutting tool, for example adiamond chisel.

The surface of a master for optical components may have a variety ofprofiles according to the shape of the surface of the products (forexample lenses, mirrors, or prisms) which are manufactured by the fathermatrix or son matrix derived from the master. The master may have, forexample, a spherical surface. Alternatively, the master surface may beaspherical.

The desired profile is provided by precision machining, for examplesuperfine milling or turning by a diamond-edged tool. As a result ofthis treatment, the surface of the electroplated aluminum obtains aperfect optical quality in contrast with aluminum obtained differently.The surface has a supersmooth texture having a surface roughness (R_(T))of at most 0.02 μm. The surface does not show any micropits. The surfaceroughness, R_(T), denotes the maximum peak-to-valley height.

It is known per se from Netherlands Patent Application No. 7812062 tomanufacture a supersmooth surface by a metal-removing treatment ofelectroplated aluminum. As a result, high grade mirrors, in particularinfrared mirrors, can be made. In contrast with the present invention,the Netherlands Patent Application does not relate to the manufacture ofa master and the associated problems of the manufacture of electroplatedmetal copies and the detaching of the copies from the mold whilemaintaining the optical surface quality of both the master and of themetal copy.

According to the invention, the profiled surface of the master comprisesa layer of titanium nitride. Due to the thickness of the titaniumnitride layer, the profile of the surface of the layer of electroplatedaluminum will change to a small extent. When making masters used in themanufacture of optical components, a suitable correction should be madein the precision treatment of the electroplated aluminium. The thicknessof the titanium nitride layer is preferably 10-300 nm.

The titanium nitride is preferably provided by means of a reactivemagnetron sputtering process as is described in the article by Aronsonet al entitled "Preparation of Titanium NItride by a Pulsed D.C.Magnetron Reactive Deposition Technique Using the Moving Mode ofDeposition" (Thin Solid Films, Vol. 72, pp. 535-540, (1980). Thereactive gas is nitrogen, the plasma is argon and the target istitanium.

The titanium nitride layer adheres excellently to the electroplatedaluminum layer and readily follows the profile. The quality of theadhesion and the build up of the titanium nitride layer is so good thatthe exposed surface of the titanium nitride layer has the same opticalproperties as the electroplated aluminum layer.

Titanium nitride is an extremely hard and wear-resistant material havingexcellent scratch resistance. At normal temperature the material is notsensitive to oxidation in air and is chemically stable. For example, thematerial is not attacked by water or by aqueous solutions of weak acidsand bases.

The titanium nitride is an electric conductor so that a metal, forexample Ni or Cu, can be deposited thereon directly by electroplating.For manufacturing a negative metal copy, for example a Ni copy, themaster is placed in an electroplating bath. The bath comprises, forexample, an aqueous solution of nickel sulphamate.

The cathode of the bath is connected to the layer of titanium nitride.When current passes through the baths a Ni peel is deposited on thetitanium nitride.

The Ni-peel can easily be detached from the layer of titanium nitridewithout damaging the titanium nitrode layer. The quality of the masteris therefore maintained so that it can be used again for the manufactureof other metal copies.

The deposition of metal on the surface of the titanium nitride and thesubsequent detaching of the resulting metal peel are so good that thesurface of the metal peel (the father matrix) has the same orsubstantially the same optical quality as the surface of the titaniumnitride layer.

Further copies of the resulting father matrices can be made, forexample, into mother matrices and son matrices. Synthetic resin copiesare made from the son matrices, for example, by using a compressionprocess or a molding process. The father matrix may also be used formaking synthetic resin copies. All of these copies have excellentsurface quality which corresponds to that of the master.

It has surprisingly been found that when a layer of titanium nitride isused having a thickness less than or equal to 300 nm, the layer ofmetal, for example Ni, deposited on the layer of titanium nitride byelectroplating has the same properties (such as structure and grainsize) as a layer of Ni which has been provided directly on a surface ofelectroplated aluminum.

The excess voltage required to deposit Ni by electroplating is also thesame for titanium nitride as for aluminum. The excess voltage is definedby the current strength equation i=i_(o) e^(-kn) where

i=required current density,

i_(o) =exchange current density,

k=constant, dependent on the metal which is deposited by electroplating,and

n=excess voltage.

The excess voltage is determined by various factors, such as diffusionof metal ions in the metal plating bath, the dehydration of the hydratedmetal ions, charge transfer, nucleation, and so on.

The titanium nitride layer, in a thickness ≦300 nm, provided onelectroplated aluminum does not influence the electroplating of metal,in particular Ni. The characteristics of the deposited metal layer arenot influenced by the titanium nitride layer, either. The electroplatedmetal layer can be detached from the surface of the titanium nitridewithout any problems.

In a further preferred embodiment of the master according to theinvention, a layer of titanium is present between the layer ofelectroplated aluminum and the layer of titanium nitride. The thicknessof the titanium layer is, for example, 2-8 nm. The layer can be providedin a simple manner by a small adaptation of the reactive sputteringprocess used to provide the titanium nitride. By withhelding nitrogen inthe first phase of the sputtering process, titanium will be deposited.The plasma is argon and the target is titanium.

The layer of titanium improves the bonding of the titanium nitride layerto the layer of electroplated aluminum if the aluminum has an oxideskin. During the mechanical treatment of the electroplated aluminumwhich usually takes place in ambient air, an oxide skin will be formed.If the oxide skin is removed, no intermediate layer of titanium isnecessary. However, since the removal of the oxide skin requires anextra process step, it is preferred to leave the oxide skin and toprovide an intermediate layer of titanium in order to obtain a good bondbetween the titanium nitride and the aluminum.

If no intermediate layer of Ti is provided, a titanium nitride-oxidelayer will be formed upon providing titanium nitride. If a metal, forexample Ni, is grown on this layer by electroplating, reduction totitanium will take place. The grown metal peel will not work loose fromthe titanium surface.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a master according to the inventionused in the manufacture of a negative metal copy (father matrix).

FIG. 2 is a cross-sectional view of the father matrix obtained by usingthe master of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, denotes a preform 1 is manufactured from analuminum alloy of the following composition in % by weight.

Mg 0.5-2.0,

Si 0.5-1.5,

Mn 0.2-1.5,

Fe≦0.4,

Cu≦0.5,

other impurities ≦0.1,

Al for the remainder.

Preform 1 has a spherical surface 2 on which an aluminum layer 3 havinga thickness of 50 μm is deposited electrolytically (by electroplating).The electrolysis bath used for this purpose comprises a solution of 1.2molar AlCl₃ and 0.2 molar LiAlH₄ in tetrahydrofuran. The electrolysis iscarried out with a current density of 1A per dm². The hardness of thelayer of electroplated aluminum is 70 Vickers.

The electroplated aluminum layer 3 is turned on a precision lathe havinga diamond chisel. An aspherical mirror surface 4 is obtained having asurface roughness, R_(T), of approximately 0.01 μm.

The surface 4 is degreased ultrasonically. The preform is then providedin a sputtering device and is subjected to a reactive triode sputteringprocess carried out at a voltage of 1.6 kV required to make a plasma. ATi target and an argon plasma at a pressure of 22.6×10⁻² Pa are presentin the device. After a short period of time, N₂ gas is admitted to thedevice until a partial pressure of 4×10⁻² Pa of nitrogen is present. Asa result of the sputtering process, a thin 5 nm layer of Ti, not shown,is deposited on the electroplated aluinum layer 3, and a titaniumnitride layer 5 having a thickness of 90 nm is provided on the Ti layer.

The exposed surface 5a of the titanium nitride layer forms theduplication surface of the master. This surface is supersmooth and has asurface roughness, R_(T), of 0.01 μm.

A Ni copy (father matrix) is manufactured from the master thus obtainedby electroplating. For this purpose, the master is placed in a nickelsulphamate bath and is connected to the cathode. Upon current passage,Ni is deposited on the titanium nitride layer 5. The resulting nickellayer 6 has a thickness of 5 mm. The layer 6 is ground and is providedwith a supporting plate 7 of stainless steel which is adhered to theground surface of layer 6.

The resulting father matrix 6 and 7 is detached from the master withoutany problems. The titanium nitride layer 5 remains entirely intact sothat the master can be used again for manufacturing metal copies.

The father matrix 6,7 shown in FIG. 2 has an aspherical surface 8 whichis the negative of the surface of the master. The surface 8 has aperfect optical quality with a surface roughness, R_(T), ofapproximately 0.01 μm.

What is claimed is:
 1. A duplication master for manufacturing negativecopies by electroplating a metal onto a duplication surface of themaster, said master consisting essentially of:a preform having asurface; a layer of electroplated aluminum on the preform surface, saidaluminum layer having a shaped surface opposite the preform surface,said shaped surface having a desired profile; and an outer coating layeron the aluminum layer, said outer coating layer having an exposedsurface which is the duplication surface of the master, said outercoating layer consisting essentially of titanium nitride.
 2. Aduplication master as claimed in claim 1, wherein the shaped surface ofthe aluminum layer is given its desired profile by precision machining.3. A duplication master as claimed in claim 2, wherein the thickness ofthe titanium nitride layer is between 10 and 300 nanometers.
 4. Aduplication master as claimed in claim 3, wherein an intermediate layerof titanium is provided between the electroplated aluminum layer and thetitanium nitride layer.